The present invention is related to an apparatus for positioning a cuvette in an optical beam path of an optical measuring instrument.
For the analysis in spectrometers or photometers or other optical measuring instruments, liquid samples are filled into cuvettes. The cuvettes have planar parallel side walls for the passage of the light beam. They have to be positioned very accurately in the optical beam path of the optical measuring instrument in order to obtain the measuring results with small systematic and random measurement deviations. For this purpose, the optical measuring instruments have cuvette shafts, (called only “shafts” in the following, where shaft is defined as a passageway for axially receiving a cuvette), through which the optical beam path passes through crosswise. By axially inserting them up to the bottom of the shaft, the cuvettes can be brought into the desired position in the optical beam path. Mostly, the shaft has a rectangular cross section for accommodating cuvettes with a box-like cross section. Most current spectrometers and photometers have a shaft with a cross section of 12.3 mm×12.5 mm. The heights of the light beam above the bottom of the shaft vary from 8.5 mm to 20 mm, depending on the apparatus. In “standard cuvettes”, the cross section and the height are adapted to the above mentioned dimensions of the shafts of most of the current optical measuring instruments.
Cuvettes for one single use (“single-use cuvettes”) have strong deviations in dimensions and weight. This applies for the outer dimensions in particular. These deviations negatively affect the accuracy of the positioning of the cuvettes in the shaft of an optical measuring instrument.
In a known photometer, a metal-made roller is pressed against an edge of the cuvette in a passage opening of the wall of the shaft by a spiral spring. Through this, the cuvette is set into a defined bearing against the wall of the shaft, and the precision of the positioning in the optical beam path is improved. When the cuvette is introduced into the shaft, the roller is deflected somewhat laterally against the elastic restoring force of the spiral spring. In a manual operation, this requires a certain expenditure of force by the user. Furthermore, the spring force may deform the cuvette, through which the positioning accuracy can be affected. With cuvettes made of plastics, wear by friction may occur in addition. Cuvettes made of quartz glass are brittle and they may crack out or break due to the load.
Starting from this, the present invention is based on the objective to provide an apparatus for positioning a cuvette in an optical beam path of an optical measuring instrument, which facilitates the use, improves the positioning accuracy and imparts fewer loads to the cuvettes.